This invention is drawn to the field of parametric acoustic sources, and more particularly, to such sources which are saturation limited.
A directional sonar source operating parametrically in a nonlinear medium at selected primary frequencies results in a directional difference frequency beam of sonic energy having low sidelobes. But as the electrical drive level of the primaries is increased, the acoustic amplitude of the difference frequency does not increase indefinitely. The farfield difference frequency amplitude progressively levels off and approaches a limiting value practically independent of the drive level. Such sources are saturation limited. With further increase in drive level above the limit of saturation, energy at the primary frequencies is converted into energy at harmonic frequencies forming multiple shock waves which are rapidly dissipated in the medium and lost as heat.
A known technique for attempting to increase the farfield amplitude of the difference frequency beam of such saturation limited parametric sources, reported by Muir et. al. at 62 J. Acoust. Soc. Am 271 (1977), launches a multifrequency saturation limited primary beam from a parametric array toward a pressure release reflector. The primary beam distorts as it propagates generating repeated shocks. Upon reflection from the pressure release reflector, a 180.degree. phase reversal creates unstable shocks that subsequently undistort such that most of the energy of the harmonic components of the shock waves returns to the primaries providing higher sound pressure levels at remote field points before the limit of saturation takes effect. The difference frequency sound beam likewise undergoes a 180.degree. phase shift at the pressure release reflector; however, the phase shifted difference frequency beam destructively interferes with the difference frequency beam parametrically produced before reflection causing a reduction in the amplitude of the difference frequency beam. Thus it is desireable to provide a means and method for selectively increasing the farfield difference frequency amplitude of parametric sonar sources which are saturation limited.